1. Field of the Invention
The present invention relates to a plasma processing system. In particular, the present invention relates to a high-frequency measurement unit and an electrical characteristic adjustment apparatus (for e.g. automatic impedance adjustment) that constitute a plasma processing system.
2. Description of the Related Art
FIG. 7 is a block diagram of a conventional plasma processing system. The plasma processing system is employed for executing an etching process or the like on a work such as a semiconductor wafer or a liquid-crystal substrate. The plasma processing system includes a high-frequency power unit 10, an impedance matching device 20, and a plasma processing unit 30. During the plasma processing, a mismatch of impedance emerges between the high-frequency power unit 10 and the plasma processing unit 30, because of large fluctuation in impedance of the plasma processing unit 30. The impedance matching device 20 serves to eliminate such disadvantage, thereby achieving impedance matching between the high-frequency power unit 10 and the plasma processing unit 30.
For monitoring the fluctuation in impedance, a high-frequency detector may be provided between the impedance matching device 20 and the plasma processing unit 30. The high-frequency detector detects a high-frequency voltage, a high-frequency current and so on.
JP-A-H10-185960 discloses a high-frequency signal-detecting probe, as an example of the high-frequency detector. The high-frequency signal-detecting probe includes an input/output terminal for high-frequency signals, a sensor that detects the high-frequency voltage, a sensor that detects the high-frequency current, an output terminal for detection signals output by those sensors, and a case that accommodates those components. The terminals are constituted of a coaxial connector.
In the case where the impedance matching device 20 and the plasma processing unit 30 shown in FIG. 7 are connected via the coaxial connector, the high-frequency signal-detecting probe can be applied. In some plasma processing systems, however, the impedance matching device 20 and the plasma processing unit 30 are connected via a waveguide for reducing transmission loss, so that the energy is transmitted in a form of an electromagnetic wave. Accordingly, the high-frequency signal-detecting probe cannot be connected to such a plasma processing system. In such a case, a high-frequency measurement unit may be provided instead of the high-frequency signal-detecting probe. The high-frequency measurement unit is attached to a position close to the output terminal inside the impedance matching device 20. The high-frequency measurement unit detects a high-frequency voltage (root mean square value, hereinafter RMS value) V, a high-frequency current (RMS value) I, and a phase difference θ between the high-frequency voltage V and the high-frequency current I.
The impedance Z of the plasma processing unit 30 can be calculated through the following formulas based on the measured values V, I, θ:Z=R+jX, R=(V/I)cos θ, X=(V/I)sin θ
Since the measurement point is close to the input terminal of the plasma processing unit 30, the impedance Z thus calculated may be construed as the impedance of the plasma processing unit 30.
In general, when measuring a physical quantity with a measurement unit that includes a sensor, the measurement unit has to be calibrated. This is because a plurality of sensors of the same type presents, from a strict viewpoint, non-ununiform sensitivities. The sensor outputs containing an error have to undergo, prior to being handled as a measured physical quantity, a calibration calculation based on calibration data obtained in advance with respect to each of the sensors, for eliminating the error.
In the plasma processing system also, the calibration calculation is required. For example, when measuring the RMS value V of the high-frequency voltage, a calibration coefficient is acquired in advance of actually activating the plasma processing system. The calibration data is acquired as follows. Firstly, a dummy load is connected instead of the plasma processing unit 30. The dummy load has the same impedance as the characteristic impedance (for example, 50Ω) of the plasma processing system. Then the high-frequency power unit 10 is set to output a predetermined power. Theoretically, the RMS value Va of the voltage obtained upon applying the power thus set to the dummy load is known. Under such state, the high-frequency measurement unit actually measures the RMS value Va′ of the voltage. Accordingly, a calibration coefficient Ca (=Va/Va′) is acquired based on the theoretical value Va and the voltage value Va′ obtained through the measurement with the dummy load. Thus, when actually utilizing the plasma processing unit, a correct voltage value can be obtained by multiplying the measured RMS value of the voltage and the calibration coefficient Ca.
Those conventional plasma processing systems employ a fixed frequency such as 2 MHz or 13.56 MHz. During the plasma processing, however, the transition of status with time of the work or the plasma often provokes fluctuation in impedance of the plasma processing unit 30. To absorb such fluctuation, recently a system has been developed that employs the high-frequency power unit 10 instead of the impedance matching device 20, to achieve the impedance matching. For example, an example of the plasma processing systems that have been put to practical use varies the output frequency of the high-frequency power unit 10 in a fine increment, to match the impedance.
The calibration coefficient has to be acquired according to the frequency. The conventional high-frequency measurement unit only bears the calibration data with respect to the fixed frequency. Accordingly, applying the conventional high-frequency measurement unit as it is to the new system can only lead to deviation of the system frequency from the frequency intended by the calibration coefficient stored in the high-frequency measurement unit. This naturally leads to a measurement error.
A possible solution of the measurement error would be acquiring the calibration coefficient with respect to the entire frequency range settable by the high-frequency power unit 10, and store such calibration coefficients in the unit. In this case, however, a huge number of calibration coefficients have to be acquired, and a much larger storage capacity has to be secured for those coefficients. Besides, the job of acquiring such large calibration coefficient group constitutes a heavy burden in establishing the system.
FIG. 14 illustrates another conventional plasma processing system. The plasma processing system includes a high-frequency power unit 100, an impedance matching unit 200, and a plasma processing unit 300. When performing the plasma processing, the impedance of the plasma processing unit 300 largely fluctuates. The impedance matching unit 200 serves to offset the fluctuation to thereby keep the input impedance Z1, i.e. the impedance at the input terminal 200a on the side of the plasma processing unit 300 generally at 50Ω.
The impedance matching unit 200 includes an input-side detector 201, a control unit 202, variable capacitors VC1, VC2, and an inductor L1. The input-side detector 201 detects a voltage Vi and a current Ii of the input terminal 200a, and a phase difference θi between the voltage Vi and the current Ii, and provides the detected value to the control unit 202. The control unit 202 includes a microcomputer, and calculates the input impedance Z1 based on the received values Vi, Ii, and θi. The control unit 202 adjusts the variable capacitor VC1, VC2 through a predetermined step to set the input impedance Z1 at 50Ω. The variable capacitors VC1, VC2 may include a mechanical adjustment unit for example, so that when an actuator controlled by the control unit 202 moves the adjustment unit the capacitance of the variable capacitors VC1, C2 varies.
Thus, the impedance matching unit 200 varies the capacitance of the variable capacitors VC1, VC2 according to the fluctuation in load impedance ZL, i.e. the impedance of the plasma processing unit 300, thereby matching the input impedance Z1 of the impedance matching unit 200 and the output impedance Zout (50Ω) of the high-frequency power unit 100.
The capacitance range achieved by the variable capacitors VC1, VC2 has a certain limit. Accordingly, there is a limit on the fluctuation range of the load impedance ZL of the plasma processing unit 300 that the impedance matching unit 200 can cope with. Therefore, prior to the actual use of the plasma processing unit 300, it has to be examined under which range of the load impedance ZL the impedance matching unit 200 is capable of achieving the impedance matching.
FIG. 15 illustrates a system that examines the operation of the impedance matching unit 200 for the foregoing purpose. This system employs, instead of the plasma processing unit 300, a dummy load 400 that simulates the load impedance ZL of the plasma processing unit 300.
The dummy load 400 includes an inductor L2, variable capacitors VC3, VC4, and a terminal resistance R1. The variable capacitors VC3, VC4 include a mechanical adjustment unit, so that moving the mechanical adjustment unit causes variation by stages of the capacitance of the variable capacitors VC3, VC4. Varying the capacitance of the variable capacitors VC3, VC4 allows setting the impedance of the dummy load 400 as desired. The terminal resistance R1 has the value of 50Ω that is the same as the characteristic impedance of the system.
The examination of the operation of the impedance matching unit 200 is executed, for example, as follows. Firstly, 40 representative values are selected out of the fluctuation range of the load impedance ZL of the plasma processing unit 300. The impedance of the dummy load 400 is set at one of the 40 values. It is then examined whether the input impedance Z1 intrinsic to the impedance matching unit 200 matches the output impedance Zout of the high-frequency power unit 100. This step is executed with respect to all the 40 values. Such procedure allows examining whether the impedance matching unit 200 is capable of achieving the impedance matching throughout the assumed frequency range. However, such examination takes a significantly long time.
Such procedure further requires that the operation of the dummy load 400 be confirmed in advance of the examination of the operation of the impedance matching unit 200. In other words, in order to set the load impedance ZL of the dummy load 400 at a predetermined value, it has to be checked in advance how the adjustment mechanism of the variable capacitors VC3, VC4 should be moved. Hereinafter, the adjustment status of the variable capacitors VC3, VC4 will be referred to as “adjustment position C3, C4”.
The impedance adjustment of the dummy load 400 may be performed as follows. Firstly, the load impedance ZL to be reproduced by the dummy load is determined. Then the dummy load 400 is connected to the impedance measurement unit, and the impedance Z=R+jX of the dummy load 400 is measured. From the measured impedance Z, the real part R and the imaginary part X are led out, and plotted on a Smith chart, to check a shift between the target load impedance ZL and the measured impedance Z. Then the adjustment positions C3, C4 of the variable capacitor VC3, VC4 are varied according to the shift, and then the impedance Z of the dummy load 400 is again measured. The variation of the adjustment position and the measurement of the impedance Z are repeated, until the measured impedance Z enters a tolerance range. Such process can only be performed by experts versed in the relationship between the adjustment positions C3, C4 and the impedance Z.
Referring to the Smith chart shown in FIG. 8, a process of the impedance adjustment of the dummy load 400 will be described. The impedance measured at the beginning is indicated by point A. The target impedance is indicated by point B. Changing the adjustment position C3 of the variable capacitor VC3 by a predetermined amount causes the impedance of the dummy load 400 to move from A to C. Further, changing the adjustment position C4 of the variable capacitor VC4 by a predetermined amount causes the impedance of the dummy load 400 to move from C to D. It is difficult, even for the experts of this operation, to predict how the impedance moves upon changing the adjustment positions C3, C4 of the variable capacitors VC3, VC4. Accordingly, it even takes several hours to perform the impedance adjustment with respect to 40 impedance values. Moreover, the load impedance ZL also fluctuates owing to a shift in adjustment of the variable capacitor caused by switching on or off the dummy load 400, individual difference and deterioration with time of the variable capacitor, as well as to conditions such as a temperature and nature of the cooling water. Therefore, although accurate adjustment positions are once examined and recorded, when the dummy load 400 is employed in a subsequent process the accurately identical impedance cannot be reproduced. Consequently, the impedance setting, which consumes a considerable time, has to be executed immediately before each use of the plasma processing system.